There is a need for speed control for boats and a specific example is during fishing by trolling, wherein it is highly desirable to be able to troll at selected substantially constant speeds. Known boat speedometers are judged by some not to have the resolution needed in the trolling speed range. Prior attempts have been made to provide speed indicating devices of the general type disclosed, as exemplified by U.S. Pat. Nos. 2,152,768, 2,707,878 and 3,871,219. There are deficiencies and disadvantages in each of these prior art devices when compared to the features of the present invention, as will be more apparent from the following description.